Also the time constants for any filtering will not be correct. The audio regenerated for the audio stages is not suitable as it has often been passed through a capacitor to remove the DC level. For AM it is necessary to ensure that the carrier level is detected and not the modulation level. Demodulator output: the most widely used sources of the AGC voltage are from the demodulator.There are several options that can be used: Typically the control voltage for the AGC is derived from the last stages of the receiver. The implementation of an automatic gain control will depend on the design of the radio and its overall capabilities. Also if any stages of the receiver became overloaded then any amplitude information could become distorted. This can lead to issues like desensitisation, cross modulation and even the reception of spurious signals generated in the receiver if the RF or mixer stages overload. The other requirement is that it is possible that when receiving strong signals, some stages of the receiver become overloaded. As signal can vary over a range of more than 70 or 80 dB, the audio needs to be controlled accordingly. With many superhet radios being used to receive amplitude signals like AM or SSB, some control over the final output is needed otherwise the audio output will vary widely. Two of the main requirements of an AGC system in a superheterodyne receiver are that it should ensure that the audio or other output does not vary over such a wide range that the audio output continually needs adjusting. Unfortunately this is not always easy to achieve and sometimes a poorly implemented AGC can detract from the reception under some circumstances. AGC requirementsĪn automatic gain control needs to be implemented so that it enhances the reception. Whilst the automatic gain control still serves to control the output volume, a well designed AGC system will also help ensure that the receiver does not become overloaded in the presence of strong signals. To combat these types of issue, the automatic gain control or automatic volume control was introduced - the term automatic volume control, AVC being used considerably less widely these days. Also when using a radio in a vehicle on AM, large variations in signal strength can occur. Large variations in signal caused by fading on the medium wave broadcast bands or on the short wave bands can lead to large changes in volume at the output of a radio if no action is taken. Accordingly the AGC was also often called an automatic volume control or AVC.
The automatic gain control, AGC, was introduced to stop variations in signals causing large variations in the received volume.
Superhet radio Superhet theory Image response Block diagram / overall receiver Design evolution Double & multi-conversion superhet Specifications Superhet Radio AGC - Automatic Gain Control The automatic gain control, AGC within a superhet radio enables the gain of the receiver to be controlled to level the audio output, but to prevent overloading.īlock diagram / overall receiver RF amplifier & tuning RF mixer IF amplifier & filter Automatic gain control, AGC